Assimilate uptake and the regulation of seed development

Seed development is a series of events involving cell division, followed by cell differentiation and storage activity. In legume cotyledons, cell differentiation starts in certain regions and gradually spreads to other parts, thereby building up developmental gradients. The entire process appears to be subject to metabolic control. The high hexose state of the premature legume embryo as controlled by seed coat-specific invertases favours cell division. Differentiation is initiated when hexose decreases and sucrose increases. Seed development occurs in a close interaction with seed metabolism and transport processes. Movement of photoassimilates from the sieve tubes to the unloading region of the maternal seed tissue is symplasmic and controlled by plasmodesmal passage. Sucrose uptake into Vicia faba cotyledons is mediated by a H+-sucrose symporter located in the outer epidermis which generates transfer cells. Formation of the sucrose uptake system is induced during the early to mid-cotyledon stage by tissue contact with the maternal seed coat and is controlled by carbohydrate availability. In contrast, a hexose transporter gene is also expressed in epidermal cells covering younger, mitotically active regions of the cotyledons. The sucrose uptake system apparently generates the high sucrose state immediately preceding the storage phase. Sucrose specifically induces storage associated differentiation processes indicating a specific sucrose-dependent signalling pathway operating in maturing cotyledons. Moreover, the mode of sucrose uptake - apoplasmic movement into the epidermal cells with subsequent symplasmic transfer to the storage parenchyma cells - appears to control coordinated cotyledon development. Unlike sucrose, amino acid transport into legume cotyledons is passive during Early development but at later stages when large amounts of storage proteins are synthesized an additional active uptake system is established to ensure a sufficient supply.